Gas Planning Calculator: Rock Bottom and Turn Pressure
Rock bottom is the gas two divers need to reach the surface sharing one supply: one minute at depth, an ascent at 30 feet per minute, three minutes at 15 feet, and the final ascent, all at an elevated rate. From 100 feet that is roughly 33 cubic feet, which is about 1,300 psi in an aluminium 80. Turn pressure on an out and back dive is start pressure minus half the gas above rock bottom.
Rock bottom, also called minimum gas, is the pressure at which a dive ends regardless of what you would rather be doing. It is not a guess and it is not the 500 psi the boat asks you to surface with. It is a calculated volume: the gas two divers need to get from the deepest point of the dive to the surface, sharing one supply, breathing hard, with a safety stop included. Once you have that figure, your turn pressure follows from it, and the whole dive becomes a plan rather than a hope.
A plan is only as good as the gauge you check it against. Even on an air integrated computer, a mechanical submersible pressure gauge is the backup that cannot lose a wireless link or a battery, and writing the numbers on a wrist slate before you get in removes the need to remember arithmetic at depth. Both together cost less than one weekend of boat fees.
- Reserve volume
- 33.5 cu ft
- Usable gas
- 1,700 psi
- Bottom time to turn
- 11 min
- Total bottom time
- 22 min
Where the reserve goes, phase by phase
| Phase | Duration | Average depth | Average pressure | Gas for two divers |
|---|
What is rock bottom, and why is 500 psi not it?
Rock bottom is a volume of gas calculated for a specific depth, sized so that two divers sharing one cylinder can reach the surface with a controlled ascent and a safety stop. It is depth dependent, which is the whole point: the deeper you are, the more gas the same ascent costs, because you breathe faster at pressure and the ascent takes longer.
Compare that with the flat 500 psi convention. In an aluminium 80, 500 psi is about 13 cubic feet. At 100 feet, two divers breathing hard at 1.0 cubic feet per minute each consume 8 cubic feet per minute between them. That is a little over 90 seconds of gas, and a controlled ascent from 100 feet with a three minute stop takes over six minutes. The 500 psi rule is not conservative at depth. It is roughly a quarter of what the situation requires.
The rule is not useless. In 30 feet of water it is fine, because the ascent is short and cheap. It is simply a rule that stops working exactly where the consequences start, which is the worst property a safety rule can have.
How is rock bottom calculated, step by step?
Four phases, each priced at the average ambient pressure over that phase, for two divers at an elevated breathing rate.
- One minute at depth. Time to recognise the problem, get the alternate air source into a mouth, sort out contact and start moving. This minute is bought at full depth pressure, which makes it the most expensive minute in the plan.
- Ascent to the stop at 30 feet per minute. From the planned depth to 15 feet. Priced at the average of the two depths, which is where most of the arithmetic lives.
- Three minutes at 15 feet. The safety stop. Cheap at 1.45 ata, and worth keeping in the plan rather than dropping it: a stop is more valuable after a stressful ascent, not less.
- Final ascent from 15 feet to the surface. Half a minute at 30 feet per minute, and slower is better.
Phase gas = duration (min) x average pressure (ata) x combined RMV (cu ft/min)
Worked at 100 feet with 1.0 cubic feet per minute per diver, so 2.0 combined:
- 1 min at 100 ft, 4.03 ata: 1 x 4.03 x 2 = 8.1 cu ft
- 2.83 min from 100 to 15 ft, average 57.5 ft, 2.74 ata: 2.83 x 2.74 x 2 = 15.5 cu ft
- 3 min at 15 ft, 1.45 ata: 3 x 1.45 x 2 = 8.7 cu ft
- 0.5 min from 15 ft to surface, average 7.5 ft, 1.23 ata: 0.5 x 1.23 x 2 = 1.2 cu ft
Total: 33.5 cubic feet. In an aluminium 80 at 0.0258 cubic feet per psi, that is 1,298 psi, so call it 1,300. That is the number that ends the dive.
| Depth | Reserve volume | In an AL80 | In an AL63 | In a steel HP100 | Versus 500 psi |
|---|---|---|---|---|---|
| 40 ft | 17.0 cu ft | 660 psi | 805 psi | 585 psi | 1.3x |
| 50 ft | 19.6 cu ft | 760 psi | 928 psi | 675 psi | 1.5x |
| 60 ft | 22.4 cu ft | 868 psi | 1,060 psi | 771 psi | 1.7x |
| 70 ft | 25.3 cu ft | 981 psi | 1,198 psi | 871 psi | 2.0x |
| 80 ft | 28.3 cu ft | 1,097 psi | 1,340 psi | 974 psi | 2.2x |
| 90 ft | 30.8 cu ft | 1,194 psi | 1,458 psi | 1,060 psi | 2.4x |
| 100 ft | 33.5 cu ft | 1,298 psi | 1,585 psi | 1,153 psi | 2.6x |
| 110 ft | 36.7 cu ft | 1,422 psi | 1,737 psi | 1,263 psi | 2.8x |
| 120 ft | 39.9 cu ft | 1,546 psi | 1,888 psi | 1,373 psi | 3.1x |
| 130 ft | 43.2 cu ft | 1,674 psi | 2,045 psi | 1,487 psi | 3.3x |
Read the last column. At 40 feet the calculated reserve is a little over the traditional 500 psi and the old rule is nearly adequate. At 130 feet it is more than three times as much, and a diver following the 500 psi convention is planning to start an emergency ascent with under a third of the gas the ascent needs. That gap is the whole argument for doing this calculation rather than following a habit.
Notice too what cylinder choice does. The same 33.5 cubic feet is 1,298 psi in an aluminium 80 but 1,585 psi in an aluminium 63, because the smaller cylinder gives up more pressure for the same gas. Divers who move between rental cylinders and their own frequently get this backwards. Calculate the volume first, then convert to pressure for the cylinder in front of you. The SAC rate calculator covers the same trap from the consumption side.
How do you set a turn pressure?
Turn pressure is where the plan meets the dive. Usable gas is what you start with minus rock bottom, and how much of it you spend before turning depends on the shape of the dive.
- Out and back, free ascent available: turn at half the usable gas. You spend half getting somewhere and half getting back, and the reserve underneath is untouched.
- Fixed exit a long way off, or current that will be against you: turn earlier. Forty percent rather than fifty is a common adjustment. If the swim back is genuinely committing, treat it as an overhead problem even though there is water above you.
- Overhead environment: rule of thirds. A third in, a third out, a third untouched. Caves and wreck penetration are where this belongs, and both require specific training, redundant gas and a continuous guideline. This page describes the rule so you recognise it, not so you can dive it.
A worked example on the numbers above. Starting at 3,000 psi with a rock bottom of 1,300 leaves 1,700 psi usable. Half of that is 850, so the turn pressure is 2,150 psi. Write both numbers on your slate before you get in, along with the maximum depth and, if you are on nitrox, the mix and its maximum operating depth. Three numbers on a slate is a plan. Three numbers you intend to remember at 100 feet is not.
What does this calculation assume, and when does it break?
Every one of these assumptions is doing work, and each of them fails somewhere real:
- A direct ascent is available. False inside a wreck, under ice, and in a cave. In an overhead environment your reserve has to cover the entire exit, which is why the rule of thirds exists and why penetration requires training this page does not provide.
- Both divers are conscious and cooperative. An unresponsive diver changes the ascent completely and adds a rescue on top of it.
- No decompression obligation. If you have accumulated one, the ascent includes mandatory stops and the reserve grows accordingly. Decompression diving requires technical training, redundant gas and a plan this calculator does not produce. If you are planning dives that incur an obligation, take the course.
- The elevated breathing rate is high enough. One cubic foot per minute is a planning convention, not a measurement. A genuinely panicked diver can exceed it.
- The ascent is at 30 feet per minute. Faster is not safer, and slower costs more gas. The reserve assumes control.
None of that makes the calculation less useful. It makes it a floor rather than a ceiling. When the arithmetic and your instinct disagree about how much gas to keep, keep more.
How do you actually use this on a dive?
Before the dive: agree the maximum depth, calculate rock bottom for that depth, subtract it from your start pressure, and set the turn pressure from what is left. Write it down. Both divers use the more conservative of the two figures, because the pair turns together and the reserve has to cover the diver with less gas.
During the dive: check the gauge on a schedule rather than when you remember, roughly every five minutes and always before and after any depth change. Turn at the turn pressure, not at the interesting thing just past it. Begin the ascent at rock bottom, not below it.
After the dive: log what you actually used against what you predicted. That comparison is how your respiratory minute volume becomes a real number rather than an assumption, and it is the input that makes every future plan on this page more accurate. A written log with the conditions recorded alongside the pressures is worth more than an app export with none.
Related tools and reading
- SAC rate calculator, which produces the RMV this page needs
- Nitrox MOD calculator, for the oxygen ceiling on your mix
- Equivalent air depth calculator, for the nitrogen side
- Best dive computers, including air integrated models
- Intermediate scuba setup, the kit that supports planned diving
- Dive computer specifications A to Z
Frequently asked questions
What is rock bottom gas planning?
Rock bottom is the volume of gas two divers need to get from the deepest point of the dive to the surface while sharing one supply. It covers a minute at depth to solve the problem, a controlled ascent at 30 feet per minute, a three minute stop at 15 feet, and the final ascent, all priced at an elevated breathing rate for two people. When your pressure gauge reaches that figure, the dive is over.
How much gas do you need to reserve at 100 feet?
For two divers at an elevated respiratory minute volume of 1.0 cubic feet per minute each, an ascent from 100 feet with a three minute safety stop needs roughly 33 cubic feet. In an aluminium 80, which holds 77.4 cubic feet at 3,000 psi, that is about 1,300 psi. That is a reserve you ascend on, not a reserve you swim past on the way back to the boat.
Is the 500 psi rule good enough?
Not at depth. A flat 500 psi reserve is a boat courtesy rather than a calculation, and it ignores depth entirely. At 100 feet, 500 psi in an aluminium 80 is about 13 cubic feet, which two sharing divers consume in roughly three minutes. That is less time than a controlled ascent with a safety stop takes. The rule works acceptably in shallow water and fails exactly where it matters.
What is the rule of thirds and when does it apply?
A third of your gas out, a third back, and a third held in reserve. It is the overhead environment rule, used in caves and inside wrecks where a direct ascent is impossible, and it exists because the way out may be the same length as the way in. Overhead diving requires specific training and equipment. On an open water dive with a free ascent available, rock bottom plus a turn pressure is the more appropriate method.
Why plan for two divers rather than one?
Because the reason you would ever need the reserve is that someone has no gas, and the most likely someone is your buddy. A reserve sized for one diver leaves you both short in exactly the scenario it was calculated for. Planning for two, at a raised breathing rate, is the difference between a reserve that describes an emergency and one that describes a calm dive that never happened.
What breathing rate should I use for an emergency ascent?
Higher than your normal one. A common planning figure is 1.0 cubic feet per minute per diver, roughly 28 litres per minute, against a relaxed recreational rate of 0.4 to 0.6. A diver who has just run out of gas is not relaxed, and neither is the one donating. Using your calm rate for an emergency calculation produces a reserve that is right on paper and short in the water.
How we choose: we compare published manufacturer specifications, published training agency standards, and verified owner reviews across retailers. We do not test gear in person, and every depth rating, cylinder capacity and algorithm name quoted here is the manufacturer's published figure unless we say otherwise, so confirm it on the current spec sheet before you buy. Nothing here is dive instruction, and no calculator output on this site is a dive plan. Scuba diving carries a real risk of decompression sickness, oxygen toxicity, barotrauma and drowning. Dive within the limits of your certification, verify every plan with your own computer, and buy the training before the gear that assumes it.
Logging your own SAC rate and gas plans? The Dive Kit & Air Planning Workbook is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.